The objective of this series of experiments is to evaluate the basic mechanical properties of cardiac muscle. A preparation of long, thin rat papillary muscles is used. One end is connected to a motion generator, while the other end is hooked to a force transducer. The muscle is illuminated with a laser beam, and the resulting diffraction pattern is recorded with a linear, photosensitive charge coupled array. From the diffraction information, sarcomere length is computed, on line. In this way we are able to relate instantaneous sarcomere length, or velocity of sarcomere shortening, to load and to time after stimulation. We use this technique to study the length-tension relation, force-velocity relation, and elastic components of cardiac muscle, as well as the response to pharmacologic agents. BIBLIOGRAPHIC REFERENCES: Iwazumi, T., Byers, M., and Pollack, G.H.: Orientation and length of cross-bridges in cardiac and skeletal muscle. Biophys. Soc., February, 1976. (abstract). Krueger, J.W. and Pollack, G.H.: Maximum velocity and isometric tension relative to sarcomerelength in intact cardiac muscle. Biophys. Soc., February, 1976.